I realized that this might be the dumbest question here, but being a total beginner as I am, I really couldn't figure it out after trying all kinds of instructions I could found on the web.

Specs: Ubuntu 13.04;

What I intended to do: check out the Algorithm Competition section, by clicking the second-to-left tab,located on the top-left of the page: http://community.topcoder.com/tc

What I got: a pop-up saying

Java not found! Java could not be automatically detected on your
machine. This page will attempt to automatically install Java and Java
Web Start. If the download and installation does not occur
automatically, click the link below to go to the Sun website where you
can download the latest version of Java.

What I did: I followed instructions on this Ubuntu Documentation page: https://help.ubuntu.com/community/Java and installed OpenJDK(Java Runtime Environment/Browser plugin/SDK) through Ubuntu Software Center. Then I rebooted the system, tried the page again. But I still got the java not found pop-up described above.